If you look through kilt makers web sites you will see repeated over and over, "Do not use or send us your pants size".
Trouser today are using what is called "Vanity Sizing". This means that the manufacturer is purposly listing the sizing one to two sizes smaller than the actual measurement of the garment in an attempt to let people think they are smaller than they actually are.
This Vanity Sizing has been the norm in Womens clothing for years. (Ask your wife or girlfriend).

The other thing that is a common mistake is to measure your waist at one location but wear your kilt at another. You must measure your waist at the same location that the waistband of the Kilt will sit.

A third common mistake is not having a kilt made to the correct measurement. Did you re-measure the finished kilt after you finished it?
To measure the waist size of a kilt lay it out flat on a large table or the floor and with a flexible tailors tape measure from the right edge of the waistband across the top of the kilt all the way over to the right edge of the under apron. (Do not measure the under apron.)
If your kilt has straps and rise where the straps are below the top of the kilt the measurement must be taken at the level of the straps.

I hope this helps you to determine if the kilt is OK or if you should take it apart and re-size it. If you made the kilt yourself then you already know how it is put together and it should be a simple process to take it apart and put it back together.